Oxygen tends to move from a gas containing a high concentration of oxygen to one of lower concentration. If the two gases are separated from each other by an oxygen ion conductor, oxygen molecules will dissociate by catalysis on one surface of the conductor and absorb electrons to form oxygen ions. These oxygen ions can then diffuse through the ionic conductor, leaving the entry surface with a deficiency of electrons. Emerging on the exit or low oxygen concentration side of the ion conductor, oxygen ions give up electrons via catalysis to form molecular oxygen, leaving the exit surface with an excess of electrons. Thus, an electrical potential difference, or EMF, is set up between the two surfaces of the ionic conductor. The greater the difference in oxygen content of the two gases, the greater will be the tendency of oxygen to diffuse through the conductor, and the greater will be the potential difference between the entry and exit surfaces.
These basic principles underlie the operation of oxygen sensing devices, which are generally well known in the art. Oxygen sensors function by monitoring the EMF developed across an oxygen ion conductor which is exposed to gases having different oxygen partial pressures. The reciprocal principle underlies the operation of oxygen separators (also called oxygen generators) such as disclosed in U.S. Pat. No. 4,296,608. Voltage is applied across an oxygen ion conducting material and oxygen ions will be forced to flow across the material such that the oxygen partial pressures become equal. Thus, one gas will become richer in oxygen than the other, resulting in a basic oxygen generator. A physical structure for an oxygen generator composed of an oxygen ion conducting material is disclosed in U.S. Pat. No. 5,205,990 and is highlighted in prior art FIGS. 1-3 in the present application and the entire disclosure is hereby incorporated by reference.
Generally, a prior art oxygen generator 10 includes a ceramic honeycomb body 12 made of an oxygen ion conducting material having a first plurality of channels 14 and a second plurality of channels 16 extending therethrough from a front face 18 to a back face 20. The channels 14 and 16 are arranged in alternating rows, resembling a striped pattern laterally across faces 18 and 20 of the ceramic honeycomb body 12. The oxygen generator 10 further includes a voltage source 22 electrically connected to the channels 14 and 16 through electrode connectors 24 and 26 which are located on a top portion 28 of the ceramic honeycomb body 12, respectively. Each of the channels have electrodes disposed over their respective side walls along their entire length. The connection methodology is such that each of the first channels 14 are electrically connected in parallel and each of the second channels 16 are also electrically connected in parallel. The voltage source 22 is operable to apply a voltage across the electrodes within the channels 14 and 16, thereby creating a voltage potential across the first and second channels 14 and 16 and enabling oxygen ion conduction through the ceramic honeycomb body 12 from one channel to another.
The oxygen generator 10 receives a source gas 30 containing some oxygen, for example, air, into the first channels 14 which are open on both the front face 18 and back face 20 of the body 12. The oxygen ions pass through the oxygen ion conducting material of the body 12 from the first channels 14 to the second channels 16 which are sealed on both the front face 18 and back face 20 of the body 12. In this manner the source gas 30 contains more oxygen than an exit gas 32 from the back face 20 due to the conduction of oxygen ions into the second channels 16. The oxygen 34 within the second channels 16 is collected from a side face 36 of the body 12 via a plurality of third channels 38 which laterally intersect the second channels 16 approximately in the middle of the side face 36.
Prior art FIG. 2 is a perspective view illustrating in greater detail a prior art electrode composition wherein electrodes 40 and 42 are disposed on the surfaces (side walls) of the channels 14 and 16. The prior art electrode composition, for example, is a platinum, catalytic material. Prior art FIG. 3 is another perspective view illustrating another prior art electrode composition having platinum as the electrodes 40 in the first channels 14 and a copper, nickel wool or mesh 44 in the second channels to further reduce electrode resistance. Neither prior art electrode composition, however, is satisfactory. For example, platinum is an expensive raw material, making the cost of forming electrodes within the channels 14 and 16 approximately $300 for the generator 10 of prior art FIG. 1. Further, as stated in U.S. Pat. No. 5,205,990, the copper, nickel wool or mesh 44 of prior art FIG. 3 is quite expensive. Since oxygen generators are desired for individual, ambulatory medical care applications, such prior art electrode compositions make such product applications cost prohibitive.
In addition, the prior art electrode compositions exhibit a substantial resistivity, which causes undesirable excess power dissipation. Since oxygen generators are desirable for ambulatory care applications, it is often necessary to use oxygen generators in conjunction with batteries. Therefore the excess power dissipation associated with the high prior art electrode resistivity is undesirable. Further still, the appreciable electrode resistivity requires that the channels 14 and 16 be rather short in length to avoid current crowding along the lengths of the channels 14 and 16. Having short channels negatively impacts the oxygen generating capacity of the generator and/or places constraints on the oxygen generator's form factor for a given oxygen generating capacity. Lastly, a large number of short channel lengths make the extrusion of the body 12 more expensive and less reliable and is consequently undesirable.